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[Objective] Nitrogen is the most important nutrient element in the growth and development and yieldformation of crops. NH4+-N and NO3--N are two major nitrogen forms which could be absorbed andassimilated by crops. Effects of nitrogen forms on the growth of crops are different. Secondary soilsalinization and scarcity of water resources are the major factors which restrain the growth of cottonseedlings at the earlier stages in Xinjiang province. The present studies dealt with the effect of differentammonium and nitrate ratios on the growth and development of cotton seedlings grown hydroponicallywith salt stress to elucidate the possible mechanisms by which different nitrogen forms influence thegrowth and development at the early stages of cotton seedlings at a physiological level and to enhancesalt tolerance through regulating nitrogen nutritional status in plants.[Method ] Hydroponics experiments were done to study the effect of different ammonium and nitrateratios （NH4+/NO3-:0/100, 25/75, 50/50, 75/25, 100/0） on the growth and development of cotton （Gossypiumhirsutum L. cv. Xinluzao 13） seedlings grown with different NaCl levels （0, 100, 200mM）. When twocotyledons were fully expanded, .seedlings were allowed to grow with 1/4-strength Hoagland solution for 3days, and with half-strength Hoagland solution for further 3 days, and then with full-strength Hoaglandsolution until harvesting. Then the seedlings were exposed to salinity by adding NaCl to the growthmedium in an increment of 50 mM every 12 h, until the final concentrations of 100 and 200 mM NaCl werereached. All nutrient solutions were changed twice a week, and deionized water was added daily toreplenish the waters lost by transpiration. The solution pH was adjusted daliy to 5.8--6.0 using 0.1 mMNaOH or HCl.[Result ] 1) Under both salt stress and control conditions, leaf and stem fresh weight, leaf, stem and rootdry eight, total biomass, leaf number, stem length, stem diameter, leaf area, and leaf expansion rate andLeaf area expansion rate, stem extention rate and relative growth rate of cotton seedlings increasedsignificantly with increasing NH4+-N/NO3--N ratio. Increasing salt concentration from 100 to 200 mMNaCl caused a siginificant reduction in these parameters mentioned above.2) Under both salt stress and control conditions, total root length, root surface area, root length pervolume, root volume and root surface area per volume were higher in cotton seedlings grown with mixedammonium and nitrate supply than in cotton seedlings grown with sole ammonium or nitrate supply. Withenhancing of NH4+-N/NO3--N ratio root average diameter was diminished correspondingly. Salt caused areduction in total root length, root surface area, root length per volume, root sverage diameter, root volumeand root surface area per volume under the five nitrogen regimes tested.3) Under both salt stress and control conditions, with enhancing NH4+-N/NO3--N ratio nitrate contentin leaves, leaf stalks, stems and roots increased correspondingly; Salt stress caused a reduction in nitratecontent under the five nitrogen regimes tested. 4) Under both salt stress and control conditions, with enhancing NH4+-N/NO3--N ratio NR activitiesof cotton seedlings reduced correspondingly, but GS activities increased correspondingly.5) Under both salt stress and control conditions, nitrogen concentration and nitrogen accumulationwere significantly higher in cotton seedlings grown with commixed ammonium and nitrate supply than incotton seedlings grown with sole ammonium or nitrate supply; Salt stress reduced seedling nitrogen uptakeunder the five nitrogen regimes tested.6) Under both salt stress and control conditions, chlorophyll content, SPAD reading, Pn, Gs, Tr,PS , qP and ETR significantly increased in cotton seedlings grown with ammomium/nitrate ratios of75/25 and 100/0 （p<0.05）; but all this parameters were distinctly debased under salinity stress.7) Under control conditions, stomatal conductance was the major limiting factor to affectphotosyuthetic rate, but under salt stress conditions, non-stomatal conductance was the major limitingfactor.8) Under both salt stress and control conditions, a significant linear correlation was found betweenchlorophyll contents and SPAD readings, and between photosyuthetic rate and chlorophyll fluorescence（p<0.05） in cotton seedlings grown with different NaCl levels.9) Under both salt stress and control conditions, compared with sole ammonium or nitrate nutrition themixed ammonium and nitrate nutrition could reduce MDA content of cotton seedlings. Root activity washighest in salt-stressed cotton grown with ammomium/nitrate ratio of 75/25. Analysis by Evans blue dyeingmethods also showed that root membrane lipid peroxidation was the least severe in cotton seedlings grownwith ammomium/nitrate ratio of 75/25 under salt stress.10) Under both salt stress and control conditions, SOD, POX, CAT, APX and GR activities in cottonseedlings increased correspondingly with enhancing of NH4+-N/NO3--N ratio. Salt stress caused a reductionin SOD, POX, CAT, APX and GR activities under the five nitrogen regimes tested.conclusion In the present study, we can draw conclusions that through mixed ammonium and nitratenutrition significantly improved nitrogen uptake compared with sole ammonium or nitrate nutrition, andenhanced enzyme activities which involved in nitrogen assimilation as well. It is observed the parametersof cotton seedling photosynthetic characteristics, cotton dry weight and plant growth rate are substancialhigher under the treatment of mixed ammonium and nitrate nutrition than either sole ammonium or nitratetreatment; The mixed ammomium and nitrate nutrition obviously enthanced cotton seedling antioxidaseactivities, however reduce MDA content, therefore significantly alleviated the salt inhibtation effect tocotton seedlings.
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